This article explains how to create an internal structure for a node

1. Choose the part icon on the diagram Tool Palette.
2. Click the valid container (class or collaboration).
3. Repeat these steps to create as many participants as needed.
   Tip: Choose the part icon on the diagram Tool Palette
   while holding down the CTRL key. Each click on a valid container produces a new part.
4. Link the collaborating parts by connectors.
5. Use the Object Inspector to set up the properties of the part.